1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a sheet, and more particularly, to an image forming apparatus that blows air onto sheets so that the sheets are separated from each other and fed through the image forming apparatus.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a printer and a copying machine includes a sheet feeding device for feeding a sheet one by one from a sheet-containing portion in which a plurality of sheets are contained. As an example of the sheet feeding device, as described in U.S. Pat. No. 5,645,274, there is a sheet feeding device using air to separate and lift sheets, in which a plurality of sheets are blown upwards by blowing air to an end portion of a sheet stack supported by a lifting and lowering tray and only one sheet at a time is suctioned onto a suction conveyer belt provided above.
FIG. 13 illustrates an example of the conventional blown air sheet feeding device. As illustrated in FIG. 13, a lifting and lowering tray 12 on which a plurality of sheets S are stacked is provided in a sheet container 11. When the sheets S are set on the tray 12, positions of the sheets S are retained at an end (hereinafter referred to as a leading edge) on a downstream side in a sheet feeding direction by a leading edge regulation plate 17, and the positions of the sheets S are retained at an end (hereinafter referred to as a trailing edge) on an upstream side in the sheet feeding direction by a trailing edge regulation plate 13. Further, the positions of the sheets S are also retained at both side edges in a direction (hereinafter referred to as a width direction) orthogonal to the sheet feeding direction by side regulation plates 14.
A suction conveyer portion 20, which includes a suction conveyer belt 21 for drawing up and conveying the sheet S, and an air blowing portion 30 are provided above the sheet container 11. The air blowing portion 30 blows the air to the end part of the sheets S stack on the tray to blow the a plurality of sheets S upwards, and the air blowing portion 30 separates each of the sheets S.
The air blowing portion 30 sucks air from the direction indicated by the arrows C and blows a part of this air in the direction indicated by the arrows D, and hence a few upper sheets among the sheets stack on the tray 12 are blown upwards. In addition, the air blowing portion 30 blows another part of the air in the direction indicated by the arrows E, and hence an uppermost sheet among the sheets lifted by blown air is separated from the others. The uppermost sheet can thus be drawn up by the suction conveyer belt 21.
Frequently the sheet feeding device is adopted for a high-productive machine which is capable of feeding (seventy) A4-size or LTR-size sheets or more per minute. The tray 12 includes a mechanism in which a drive unit (not shown) lifts and lowers the tray 12 in a vertical direction while keeping the tray 12 substantially horizontal. FIG. 13 also shows the conveying portion 20 that is a circular conveyer belt 21 rotated by rollers 41, to be described in more detail later.
FIG. 14 is a plan view illustrating details of the sheet container 11. The trailing edge regulation plate 13 for regulating the trailing edge of a sheet is disposed while being movable in parallel with the sheet feeding direction indicated by the arrow H. The side regulation plates 14 and 16 for regulating the side edges of a sheet are movable in the sheet width direction indicated by the arrows V.
Thus, the trailing edge regulation plate 13 and the side regulation plates 14 and 16 are movable, with the result that a minimum-size sheet SS to a maximum-size sheet LS can be stacked and supported on the tray 12. In order not to obstruct the movement of the side regulation plates 14 and 16, the trailing edge regulation plate 13 is disposed so as to be movable only in a central part in the width direction of the tray 12.
Here, the trailing edge regulation plate 13 is provided with a trailing edge separating portion 18 capable of moving in the vertical direction for regulating a position of a trailing edge portion that is an end on the upstream side in the sheet feeding direction of the uppermost sheet Sa. The trailing edge separating portion 18 has protrusions 18D protruding from a regulation portion surface 13C of the trailing edge regulation plate 13 illustrated in FIG. 13, and for pressing the trailing edge portion of the uppermost sheet Sa from above. A separation aid sheet 18E made of a material having a high friction coefficient is glued to the lower surface side of the protrusion 18D that contacts with the sheet, for applying resistance to the upper surface of the stacked sheets.
When the uppermost sheet Sa is fed by a length L2 corresponding to the protruding length of the protrusion 18D as illustrated in FIG. 13, the trailing edge separating portion 18 is lowered so as to abut the sheet Sb immediately below the uppermost sheet Sa. In this case, because of a frictional force generated by a weight of the trailing edge separating portion 18, it is possible to prevent the second-from-the-top sheet Sb from being conveyed while the uppermost sheet Sa is being conveyed, and hence occurrence of feeding more than one sheet can be suppressed. In addition, if there is no sheet positioned on the tray, the protrusion 18D abuts a surface of the tray 12.
In FIG. 13, supporting portions 18A are provided on the trailing edge separating portion 18, so as to engage with an engaging portion 13E that is provided on the trailing edge regulation plate 13. Then, the supporting portions 18A are provided with a ball bearing or a roller having a low surface friction resistance, and hence the trailing edge separating portion 18 can be moved smoothly in the directions indicated by the arrow G in FIG. 13.
Concerning the conventional sheet feeding device of such an air feeding type, U.S. Patent Publication No. 2007/228640 describes a sheet feeding device provided with a sheet surface detection mechanism for controlling a position of the uppermost surface of sheets contained in the sheet container 11.
FIGS. 15A and 15B are diagrams illustrating a structure of the conventional sheet surface detection mechanism. As illustrated in FIGS. 15A and 15B, the sheet surface detection mechanism 49 includes a sheet surface detection sensor flag 52, a first sheet surface sensor 54 and a second sheet surface sensor 55 that are turned on and off by rotation of the sheet surface detection sensor flag 52, and a sensor flag mechanism 50. The first sheet surface sensor 54 and the second sheet surface sensor 55 are photosensors and are connected to a control device (not shown).
Here, the sheet surface detection sensor flag 52 is supported by a support shaft 53 so that the sheet surface detection sensor flag 52 is capable of swinging.
Further, the sheet surface detection sensor flag 52 is provided with a first detection portion 52B for shielding a light receiving portion of the first sheet surface sensor 54, a second detection portion 52C for shielding a light receiving portion of the second sheet surface sensor 55, and a supporting portion 52D for supporting, in a rotatable manner, the sheet surface detection member 61 to be described later. The mechanism of the sheet surface detection sensor flag 52 is shown in larger detail in FIG. 15B.
The sensor flag mechanism 50 includes a supporting member 60 having an end 60a that is retained in a rotatable manner inside a suction duct 22, and a sheet surface detection member 61 that is supported at a first end by a rotation end 60b of the supporting member 60 and at a second end by a supporting portion 52D of the sheet surface detection sensor flag 52.
The sheet surface detection member 61 is disposed below a suctioning and conveying region of the suction conveyer portion 20, in parallel to the sheets S stacked on the tray 12, and in a movable manner in the vertical direction. A distance between the upper surface of the uppermost sheet Sa that is lifted while lifting the sheet surface detection member 61 and a belt surface of a suction conveyer belt 21 is S1. In addition, the supporting member that is supported in a rotatable manner inside the suction duct 22 protrudes from retraction holes 51H1, 51H2 formed in a gap between a plurality of suction conveyer belts 21 in the sheet width direction to the lower side of the suctioning and conveying region of the suction conveyer belt 21 as illustrated in FIGS. 16A and 16B. FIGS. 16A and 16B are views from underneath the suction conveyer belt 21.
The supporting member 60, the sheet surface detection sensor flag 52, and the sheet surface detection member 61 are disposed in a line as shown in FIG. 16B. Thus, even if the sheet abuts any position in the longitudinal direction of the sheet surface detection member 61, the sheet surface detection member 61 is capable of moving vertically while keeping its parallel posture (horizontal posture) and swinging the sheet surface detection sensor flag 52.
Next, a sheet surface control operation based on detection by the sheet surface detection mechanism 49 having the above-mentioned structure will be described.
When the sheets contained in the sheet container are lifted by the lifting of the tray 12, the upper surface of the uppermost sheet Sa abuts the sheet surface detection member 61. Then, when the tray 12 is further lifted, the sheet surface detection member 61 is lifted along with the uppermost sheet Sa. When the sheet surface detection member 61 is lifted, the sheet surface detection sensor flag 52 swings the supporting portion 52D upward about the support shaft 53 as its centre.
After a specific amount of time (dependent on the speed of lifting of the tray 12 and the number of sheets in the tray), as illustrated in FIG. 17A, a distance between the upper surface of the uppermost sheet Sa that is lifted while lifting the sheet surface detection member 61 and a belt surface of the suction conveyer belt 21 becomes S1. In this state, the first detection portion 52B of the sheet surface detection sensor flag 52 shields the first sheet surface sensor 54, while the second detection portion 52C shields the second sheet surface sensor 55, and hence ON signals are output. At this time, the control device stops the tray 12 based on the ON signals from the first sheet surface sensor 54 and the second sheet surface sensor 55.
Next, when receiving a feed start signal, the control device starts the air blow and controls the air input so that the upper portion SA of the sheet stack is blown upwards as illustrated in FIG. 17B and the tray 12 is lifted or lowered, thereby the uppermost sheet Sa is blown upwards in a predetermined region.
Here, when the second detection portion 52C of the sheet surface detection sensor flag 52 shields the second sheet surface sensor 55, the ON signal is output. Then, the position at which the second sheet surface sensor 55 is turned on is set as a lower limit of the air input region. If the ON signal of the second sheet surface sensor 55 is not obtained while the first sheet surface sensor 54 is on, it is determined that the position is “too low”, and the tray 12 is lifted until the ON signal is obtained.
In addition, as illustrated in FIG. 18, when a distance between the belt surface of the conveyer belt 21 and the upper surface of the uppermost sheet Sa becomes smaller than SH, the shielding by the first detection portion 52B is cancelled, and hence the first sheet surface sensor 54 does not generate the ON signal (but rather generates an OFF signal). This position is thus set as an upper limit of the air input region. If the ON signal of the first sheet surface sensor 54 is not obtained while the second sheet surface sensor 55 is on, it is determined that the position is “too high”, and the tray 12 is lowered until the ON signal is obtained.
Such series of operations is shown in the following table.
TABLE 1First sheetSecond sheetTraysurface sensor 54surface sensor 55actionONOFFLiftONONStopOFFONLower
Thus, by lifting and lowering the tray 12 based on the signals from the first and the second sheet surface sensors 54 and 55, a position of the tray 12 can be controlled to be the position where only the uppermost sheet Sa can be separated from others and conveyed. Thus, when the suction conveyer belt 21 draws up the sheet, the sheets S can be separated and fed to the image forming portion one by one. Thus, it is possible to achieve stable feeding of sheets.
There is a case where an upward curl occurs at the end portion of the sheets stacked on the tray 12 on the downstream side in the sheet feeding direction. In this case too, as illustrated in FIG. 15A described above, the sheet surface detection member 61 abuts the sheet with the curl at the end portion on the downstream side in the sheet feeding direction. Then, the sheet surface detection member 61 that abuts the sheet changes its position in parallel vertically so as to rotate the sheet surface detection sensor flag 52. Therefore, the first sheet surface sensor 54 and the second sheet surface sensor 55 are turned on and off appropriately, and hence the above-mentioned sheet surface control is performed.
In other words, the lifting and lowering of the tray 12 is controlled so that an appropriate level (appropriate distance between the suction conveyer belt 21 and the upper sheet surface) S1 is obtained at the position where the sheet surface detection member 61 abuts the sheet. Further, the upper surface of the sheet is controlled to be the appropriate level in this way, and hence a gap is generated between the sheet end portion and the suction conveyer belt 21, and hence the separation air is allowed to enter smoothly as illustrated by the arrows in FIG. 15A. As a result, the separation air securely separates the sheet from other sheets, and hence the feeding more than one sheet or jamming of a sheet can be prevented.
It is possible to dispose the sheet surface detection sensor flag 52 and the first and the second sheet surface sensors 54 and 55 outside the suctioning and conveying region of the suction conveyer belt 21 and on the upstream side in the sheet feeding direction. In this case too, the detection can be performed on the leading edge side of the sheet S, and hence the feeding of the sheet S can securely be performed. In addition, the first and the second sheet surface sensors 54 and 55 are not disposed inside the suction duct 51 in this way, and hence it is possible to reduce a height of the suction conveyer portion 20 so that the image forming apparatus can be downsized in the height direction.
The suction duct 51 is provided with the holes 51H1 and 51H2 for housing the sheet surface detection member 61 as illustrated in FIGS. 16A and 16B described above, so as not to cause resistance against conveying the sheet when the suction conveyer belt 21 draws up the uppermost sheet. The hole 51H1 is formed in the suction duct 51 in parallel to the suctioning surface (to which the sheet is drawn up) among the plurality of suction conveyer belts 21, and the hole 51H2 is formed along a vertical wall of the suction duct 51. Further, when the suction conveyer belt 21 draws up the uppermost sheet, the drawn up sheet retracts the sheet surface detection member 61 upward to be housed in the holes 51H1 and 51H2. Thus, the sheet surface detection member 61 does not protrude downward from the suctioning surface of the suction conveyer belt 21.
The hole 51H1 is formed in parallel with the suction conveyer belt 21, and hence the hole 51H1 is covered with the uppermost sheet drawn up by the suction conveyer belt 21. Thus, air is not prone to serious leaks from the hole 51H1. In addition, the hole portion 51H2 is formed in the direction orthogonal to the suctioning surface of the suction conveyer belt 21, but when the sheet surface detection member 61 is housed in the suction duct 51, the hole portion 51H2 is blocked with the sheet surface detection member 61 itself, and hence air is not prone to serious leaks through this hole 51H2 either. As a result, though the holes 51H1 and 51H2 are formed in the suction duct 51, a suctioning force is not lowered. Thus, a feeding failure of the sheet does not occur.
In the above-mentioned conventional sheet treating apparatus and the image forming apparatus provided with the same, as illustrated in FIG. 19, the sheet surface detection member 61 is housed inside the suction duct 51 in the period while the uppermost sheet Sa is conveyed. Further, in the period while the sheet surface detection member 61 is housed inside the suction duct, a level of the sheet surface of the second sheet Sb cannot be checked.
Here, the sheet surface of the second sheet Sb can only be checked when the trailing edge of the uppermost sheet Sa conveyed by the suction conveyer belt 21 passes by the sheet surface detection member 61 and the sheet surface detection member 61 drops using its weight under gravity so as to contact with the surface of the sheet Sb.
For instance, when a sheet Sa of A4 size (having the conveying-direction length of 210 mm) is conveyed by the suction conveyer belt 21 and passes by the end portion on the downstream side in the conveying direction of the sheet surface detection member 61 (L2=10 mm in FIG. 13) and drops so as to contact with the sheet Sb, necessary time period is as follows.
It is supposed that a sheet conveying speed of the suction conveyer belt 21 is approximately 1,000 mm/sec. Then, the time period when the sheet surface detection member 61 drops and contacts with the sheet Sb is (210-10)/1,000, i.e., approximately 200 milliseconds. In addition, if the sheet Sb is blown upwards below the appropriate position by 1 mm, it takes approximately 20 milliseconds for the sheet surface detection member 61 dropping by its weight to contact with the upper surface of the sheet Sb.
In addition, if a blown-upward level of the sheet Sb is not an appropriate level, it takes time to lift the tray so that the sheet surface is lifted to the appropriate level. For instance, supposing that the lifting speed of the tray is approximately 0.1 mm/sec, it takes approximately 100 milliseconds to lift the tray to the appropriate position.
In other words, if the blown-up level of the sheet is not appropriate, time period necessary for checking the sheet surface of the sheet Sb includes time until the housed state of the sheet surface detection member 61 is cancelled, time period for the sheet surface detection member 61 to become able to detect, and time period until the sheet Sb is blown upwards to be the appropriate level. In other words, to check the sheet surface of the sheet Sb whose blown-upward level is not appropriate, it takes approximately 320 milliseconds (i.e., approximately 200 milliseconds+approximately 20 milliseconds+approximately 100 milliseconds).
Here, it is supposed that a sheet feeding device is capable of usually feeding 120 sheets of A4 size per minute. Then, time per sheet is approximately 500 milliseconds. However, if it takes approximately 320 milliseconds to check the sheet surface of the sheet Sb, productivity is lowered from approximately 120 sheets per minute (approximately 500 milliseconds per sheet) to approximately 71 sheets per minute (approximately 820 milliseconds per sheet). Further, the larger the length of the contained sheet, the longer the time period of housing the sheet surface detection member 61. Therefore, if sheets of A3 size or larger are used, the throughput of sheets is further lowered.